This invention relates to the construction and operation of superconducting rotating machines, and more particularly to torque transmission assemblies for use in superconducting motors.
Superconducting air core, synchronous electric machines have been under development since the early 1960s. The use of superconducting windings in these machines has resulted in a significant increase in the magneto motive forces generated by the windings and increased flux densities in the machines. These early superconducting machines included field windings wound with low temperature superconductors (LTS), originally NbZr or NbTi, and later with Nb3Sn. The field windings were cooled with liquid helium from a stationary liquifier. The liquid helium was transferred into the rotor of the machine and then vaporized to use both latent and sensible heat of the fluid to cool the windings. This approach proved viable for only very large synchronous motors and generators (e.g., larger than 500 MW). With the advent of high temperature superconductors (HTS) in the 1980s, investigations ensued to determine the feasibility of HTS windings in superconducting synchronous machines.
The invention features a superconducting rotating machine having a relatively compact design, while still providing a relatively high output power. In effect, the construction provides a superconducting rotating machine possessing an increased power density characteristic.
The superconducting machine is of the type having a stator assembly and a rotor assembly that rotates within the stator assembly and is spaced from the stator assembly by a gap. This arrangement can be used, for example, to produce a superconducting motor or generator.
In one aspect of the invention, the superconducting rotating machine includes at least one HTS superconducting winding assembly which, in operation, generates a magnetic flux linking the stator assembly and rotor assembly, a refrigeration system for cooling the at least one superconducting winding of the rotor assembly and the superconducting rotating machine has a torque density of approximately 75 Nm/Kg or more at 500 RPM or less, the torque density being equal to the motor shaft torque divided by the motor mass. The high torque density at low speeds is advantageous in situations where a high-speed motor would require a gearbox to reduce output speed. Gearboxes are noisy, large and expensive. For example, the present invention could be utilized to drive a ship propeller without using a gearbox, thereby saving valuable ship space and reducing overall noise.
Gap shear stress is an effective measure of the torque density of a machine. It relates machine performance to the surface area in the gap between the rotor assembly and stator assembly. In particular, gap shear stress is numerically equivalent to the machine torque divided by the area and radius of the gap. If the rotor experiences a surface shear stress equal to the gap shear stress, a torque equal to the design torque would be transmitted to the shaft of the machine. A gap shear stress characteristic in a range between 15 lbs/in2 (psi) and 100 psi while achieving the desired 75 Nm/Kg or more at 500 RPM or less torque density characteristic.
Embodiments of this aspect of the invention may include one or more of the following features. In certain embodiments, the machine has a torque density of approximately 150 Nm/Kg or more at 300 RPM or less and a gap shear stress characteristic in a range between 30 lbs/in2 (psi) and 100 psi.
The superconducting winding assembly includes a superconducting coil having a superconductor tape wound about and disposed along an axis of the winding assembly to provide a plurality of concentric turns defining an opening. Each turn of the superconductor tape has a broad surface maintained substantially parallel to the axis of the winding assembly.
In certain embodiments, the superconducting tape is wound in a racetrack configuration defining a pair of opposing arcuate end sections and a pair of substantially straight side sections. The superconductor tape includes a multi-filament composite superconductor having individual superconducting filaments that extend the length of the multi-filament composite conductor and are surrounded by a matrix-forming material.
The superconductor tape includes an anisotropic high temperature superconductor, for example, Bi2Sr2Ca2Cu3O. Alternatively, the anisotropic high temperature superconductor is a member of the rare-earth-copper-oxide family.
In certain embodiments, the superconducting winding assembly includes internal support members adjacent to and alternating with the superconducting windings to help alleviate the large bending stresses that occur within the superconducting winding assembly. For example, 40-mil thick stainless steel can be alternated with the superconducting windings. The internal support members and superconducting windings form a laminate that gives mechanical strength to the system and prevents the non-circular superconducting windings from pushing themselves apart. For example, the racetrack configuration superconductor winding will attempt to become a circular winding, pushing the substantially straight side sections away from each other. The internal support members will also be coated with a thermally conductive coating that will provide a heat conduction path to cryogenic cooling tubes located within the rotor body. For example, copper could be used to coat the internal support members.
The rotor assembly of the superconducting rotating machine is enclosed in a vacuum chamber, which isolates the cryogenically cooled superconducting windings from the surrounding components. A shaft is mounted through the rotor assembly, spaced from the rotor assembly by a gap. The shaft is mounted using tangential buckle assemblies, which allow for the transfer of rotational forces between the rotor assembly and the shaft. The shaft is also mounted using axial buckle assemblies, in conjunction with the tangential assemblies. The axial buckle assemblies secure the rotor assembly axially to the shaft. Both the tangential buckle assemblies and the axial buckle assemblies utilize thermally isolating bands to thermally isolate the rotor assembly from the shaft. The shaft would act as a huge heat sink if the cryogenically cooled superconducting windings were not thermally isolated from the warm shaft. The thermally isolating bands can be manufactured from any material with a high tensile strength and low thermal conductivity. In certain embodiments, the thermally isolating bands are from a reinforced epoxy (e.g., a para-aramid and epoxy mixture). Para-aramid is sold by E.I. duPont de Numours, Wilmington, Del. under the trademark Kevlar(copyright).
The stator assembly is manufactured utilizing diamond-shaped stator coils. The stator assembly may also include individual stator coil cooling. Each coil is wrapped with an electrically insulating material and a cooling conduit for receiving coolant from an outside source is mounted to a side of the stator coil. The electrically insulating material allows the cooling conduit, which is at ground potential, to rest against the stator coil. The cooling conduit and electrically insulated stator coil are wrapped with a thermally conductive material, which facilitates cooling from the sides of the stator coil not adjacent the cooling conduit and thereby reduces the temperature gradient in the electrically insulating material.
Utilizing the external cooling conduit and electrically insulating it from the stator coil allows fresh water to be used instead of de-ionized water and a smaller, more dense stator coil is possible because one does not have to depend on air cooling the stator assembly. In certain embodiments, two cooling conduits can be mounted on opposing parallel faces to give better cooling characteristics to the system. Also, multiple passageway conduits may be used.
In certain embodiments, the electrically insulating material may vary in thickness proportional to the voltages experienced throughout the stator assemblies. Each diamondshaped stator coil experiences varying voltage, dependent on its placement within the stator. The stator coils closest to the line voltage will require the thickest insulation. The insulation on the stator coils electrically located midway between the ends of the phase will require the thinnest insulation due to the voltage drops in preceding coils. Therefore, the thickness of the electrically insulating material decreases in steps between a maximum value at the ends of the phase and a minimum value approximately half the maximum thickness at coils electrically located midway between the ends of the phase. Varying the thickness of the electrically insulating material will help facilitate cooling, since thicker insulation will not be used where it is not needed. Similar principles but a different numerical value for minimum insulation thickness will be required for phases containing an odd number of coils.
By proper matching of the pairs of coils in a two layer winding with different insulation thicknesses, and by varying the size of wire in the coils in each pair for similar temperature rise, substantially decreased temperature rise or increased resistance to voltage breakdown may be obtained.
The superconducting rotating machine includes a cryogenic cooling system for cooling the superconducting windings. The cryogenic cooling system includes a plurality of cryogenically cooled surfaces and a cryogenic fluid transport device within the cryogenic environment that moves a cryogenic fluid between the cooled surfaces and the superconducting windings in the rotor assembly. The cryogenic fluid is transported through a closed loop system that is in fluid communication with the cryogenically cooled surfaces and in thermal communication with the rotor assembly. A rotary coupling allows the cryogenically cooled surfaces and the cryogenic fluid transport device to remain stationary while the closed loop system within the rotor assembly rotates.
In one embodiment, the cryogenic fluid transport device is a cryogenically adaptable fan. The advantage of using a cryogenically adaptable fan is that the cryogenic fluid will not experience a phase change and will not require warming the fluid to ambient temperatures. Also, a cryogenically adaptable fan is smaller and less expensive than adding compressors and heat exchangers, which require a phase change.
In certain embodiments, the cryogenically cooled surfaces are Gifford-McMahon cold heads. Pulse tubes and cryogenic refrigerators are also useful as cryogenically cooled surfaces.
Other advantages and features of the invention will become apparent from the following description and the claims.